Venus: Hothouse Planet

David Grinspoon, Principal Investigator for NASA’s Exobiology Research Program and author "Venus Revealed" and "Lonely Planets"

The planet Venus is like Earth in many ways. It has a similar size and mass, it is closer to us than any other planet, and it probably formed from the same sort of materials that formed Earth. For years scientists and science fiction writers dreamed of the exotic jungles and life forms that must inhabit Earth’s twin sister.

David Grinspoon, a research scientist at the Southwest Research Institute in Boulder, Colorado, writes in his book, "Venus Revealed," that, through the Mariner 2 and other Venus missions, "we found our ‘sister planet’ to be chemically alien, as well as hot and dry to quite unearthly extremes. With these revelations, the twin-sister imagery quickly disappeared, and the notion that ‘Venus is hell’ took hold."

Only 20 percent of the sunlight that hits Venus makes it through the cloud cover, while the other 80 percent is reflected back into space. This reduced sunlight doesn’t make Venus a cold world, however, because the thick carbon dioxide atmosphere traps the planet’s heat. This greenhouse effect on Venus is often cited as a nightmare example of what could happen to Earth if we don’t get our pollution under control.

In part 1 of this interview with Astrobiology Magazine managing editor Henry Bortman, Grinspoon explains how Venus evolved from a wet planet similar to Earth to the scorching hot, dried-out furnace of today. In part 2, Grinspoon will discuss the possibility that Venus was once an inhabited world.

AM: You’ve said there were two separate major geologic transitions on Venus that led to its present-day state.

DG: Well, the conventional view has been that there were two separate transitions, but we’re suggesting it’s one overall sequence.

The first great transition in the history of Venus was the loss of the oceans. We don’t know that Venus had oceans, but there’s every reason to believe it did. All the mechanisms that supplied Earth with its initial water supply also should have worked on Venus, whether it came in with the original rocks that formed the planet, or whether it came later with comets. Venus should not have escaped whatever it was that gave Earth its water.

AM: Even though it was hotter? Wouldn’t it all have just evaporated?

DG: It probably did start losing water immediately. But still, it’s generally believed that Venus was supplied with an amount of water that, while it may not have been exactly the same as Earth’s, should have been a substantial amount. Venus probably had liquid water for some period of time.

AM: How long would the water have lasted?

DG: That’s highly uncertain. There’s no liquid water on the surface today, but there is a trace amount in the atmosphere. There are no signs of any land forms that would make us believe that water was on the observed surface in the last billion years.

Venus models have usually assumed a runaway greenhouse. That’s been modified recently to the moist greenhouse, largely by the work of Jim Kasting and his colleagues. In the moist greenhouse, the water does not last very long. How long the water lasted is the question we’re trying to answer. A number that’s often used is 600 million years.

As a young planet, Venus was losing hydrogen rapidly to space. The oceans boiled off, and after some period of time, perhaps 600 million years, there was no surface water. Then the surface and the climate were very much in the state that we see today.

AM: So, the water was lost around 4 billion years ago, at the end of the heavy bombardment period?

DG: Yeah, perhaps around that time. Now, fast forward to more recent times on Venus. We’ve begun to understand the story of its surface evolution largely due to the Magellan mission in the 1990s. The biggest surprise of Magellan was that the surface seems like it’s all the same age. That’s what I’m calling the second great transition. Something changed on Venus 600 or 700 million years ago to make the surface all the same age.

If you use the word catastrophic it rubs some people the wrong way, but something dramatic happened on Venus which wiped out almost all signs of an older surface. The planet got re-paved, basically, 600 or 700 million years ago.

AM: Did some huge impact melt the surface? Or was it the last gasp of volcanic activity?

DG: Clearly, whatever this second great transition was, it involved massive amounts of volcanism. You can see these flows that appear to be flood basalts all over, covering 80 percent of the planet. The remarkable thing is that they seem to be all the same age. The crater density is relatively uniform and random around the planet. So the planet seems to have been flooded with basaltic lavas in a geologically short period of time, simultaneously around the planet.

Much of the surface of Venus is covered by lava flows (shown above).Credit: BNSC

Now, you talk to some geologists and they argue with that and they say, well, it wasn’t simultaneous. But looking at the map of craters on Venus, all of them seem relatively pristine, and there are no older ones. You can’t escape the conclusion that something dramatic changed on Venus at that time that had the effect of re-paving the surface.

AM: So either something occurred at that time, or something that had been going on stopped.

DG: Right, exactly. Either there was an episode of resurfacing that started and stopped rather quickly, or there was an ongoing process resurfacing the planet that suddenly ground to a halt for some reason.

There may be something episodic that happens on Venus, in contrast to Earth’s steady plate tectonic recycling. Earth’s tectonics are lubricated by water in a lot of subtle ways, but Venus is much drier and instead you could have this "stop and start" action.

Earth’s tectonic activity acts as a cooling mechanism for the interior. If Venus has episodic plate tectonics, where nothing happens for a while, the heat builds up in the interior. Eventually it can’t stand it any more, and you have this rapid overturning. Then it’s quiescent for a while, and the heat builds up again. If you believe that episodic model, then the visible surface we see on Venus is the record of the last time that happened, which is maybe 600 million years ago.

Alternatively, there is the idea that Venus was continuously active and had something more like Earth-style plate tectonics, and then finally the interior cooled off enough so that it shut down 600 or 700 million years ago.

AM: And are both of those part of the conventional view? Or is one part of the new view and one part of the conventional view?

Venus up-close, as photographed by the Soviet Venera 13 lander, which parachuted to the Venusian surface on March 1, 1982Credit: Venera

DG: No, both of those are in play. In that sense they’re both conventional. Each has its advocates, and there’s isn’t any kind of surefire evidence that nails down one or the other. One of the reasons we’re advocating for a new Venus mission is to try to get the isotopic data and the surface mineralogical and other data that might help us decide between competing scenarios.

We’ve been taking a look at the models that have been done of the runaway greenhouse and the moist greenhouse to try to understand the time scale for the loss of the oceans. The first thing you realize when you look at these models is that it has not been done in a very sophisticated way. Not because the people that have done it are unsophisticated — Jim Kasting is the best in the business, and his models are state of the art. But the state of the art is not that good.

If you read Kasting’s paper, there are these huge uncertainties in the time scale. He’s had to make many simplifying assumptions to try and solve the problem of the loss of oceans on a planet like Venus. When you include all these assumptions, the real range of uncertainty in his model is longer than the age of the solar system. In other words, Venus could have lost its oceans in 10 million years, or retained them for longer than the age of the solar system. The time constraints are not that good.

So, how can one do a better job at modeling the longevity of oceans on a Venus-like planet? I say ‘Venus-like planet’ because the problem is applicable not just to Venus, but to terrestrial planets on the inner edge of the habitable zone anywhere in the galaxy, or other galaxies.

People tend to think of the habitable zone as this range with clear boundaries, and outside that line you don’t have liquid water, and inside that line you do. But in reality, it’s not going to be a clear line. As you move further away from the sun within that habitable zone, you can have oceans for longer. And the longer you have them, presumably the greater chance you have for the evolution of complex life.

Jim Kasting (above) proposed a decade ago that Earth gained an oxygen-rich atmosphere because molecular hydrogen belched out by volcanoes diffused into space.Credit: Penn State University

We decided, and Jim Kasting agrees, that the major uncertainty in the models is the role of clouds. Kasting’s models did not include clouds, not because he didn’t think of them, but because clouds are hard to model. We don’t understand how they work on a planetary scale. But it’s tempting to try to include them because of the idea of cloud feedback.

There are global-scale climate feedbacks involving clouds that could stabilize oceans and cause them to last longer on a Venus-like planet. The more water you have, the cloudier a planet is. The cloudier a planet is, the more radiation it reflects to space, and that cools things. So that tends to work opposite to a runaway greenhouse, which makes things hotter if you have more water.

In the greenhouse-era Venus, Venus still has surface water, and the atmosphere is largely water vapor. The oceans are evaporating, hydrogen is being lost to space. When we put in clouds in our model, we found that the clouds act to cool the planet significantly during that greenhouse phase. Temperatures are significantly lower.

Let me stress that this is very preliminary; it’s a work in progress. But I think our results suggest some intriguing possibilities that we now want to pursue with more rigorous models. If these results pan out, it might lead to the conclusion that liquid water on the surface of Venus lasted significantly longer. I can’t put a precise number on it yet, but it may go from hundreds of millions of years to billions of years.

If the liquid water on Venus lasted not for 600 million years, but for a couple of billion years, then I think we can start to see a scenario where the two great transitions are really one sequence.

When Venus had surface water, let’s say you also had plate tectonics. Once you lose surface water, then subduction is no longer returning hydrated silicates to the mantle, as it does on Earth, so the mantle of Venus starts to dry out. You’re no longer getting recharged with water through global tectonic cycling. It takes a while, because interior convective cycles typically have time scales of hundreds of millions of years. But after a few of those cycles, the mantle of Venus starts to become desiccated.

So as the mantle becomes desiccated, at some point that shuts off plate tectonics. Plate tectonics on Earth depends on a wet interior in several ways, largely because you have this zone of low viscosity at the base of the lithosphere on which the plates are sliding around. Water lubricates plate tectonics. You remove the hydrated minerals from the interior, and that’s going to stop. Things block up and you can no longer have plate tectonics. So if the water on Venus really went away a couple of billion years ago or less, then the drying of the interior that results from that eventually shuts down plate tectonics. The last gasp of this shutting down may have been this global resurfacing that we see evidence for in the Magellan images.

AM: So after Venus lost its water, tectonics shut down and the surface of the planet was resurfaced by lava one last time. Did this resurfacing occur because there was all this interior heat, and the normal way of releasing it was no longer there, so it all just spewed out?

DG: To me, the Magellan images don’t suggest that plate tectonics just stopped. It may be that Venus used to have something like terrestrial-style plate tectonics which was lubricated by water, and that once the water went away, it switched to a more episodic kind of behavior. And then what we’re seeing on Venus is the evidence of the last of those great episodes of global resurfacing.

AM: What are you planning to do to continue this work?

DG: Our cloud model was just a quick and dirty model. The way we handled the radiative transfer, which is the way infrared and visible radiation pass through different layers of the atmosphere, was very crude. We used what is called a gray model, which doesn’t break the spectrum into lots of separate bands, but tries to average it over the entire infrared spectrum. So one of the next steps would be to do a more sophisticated radiative transfer model, where you analyze the radiation in each separate wavelength band to get a better understanding of the temperature structure of the atmosphere in these cloudy conditions.

The cloud model is very simple, because once you get to an altitude in the atmosphere where the vapor pressure reaches saturation, we just assume a cloud forms. We also assume the particle sizes in the clouds are all the same. In reality, clouds are complex. They have multiple particle sizes, and you have things like super saturation.

Particle size distribution may sound very arcane, but it affects the way clouds impact the radiation, both coming in from the sun and going back out. We have to know what’s happening to the radiation if we’re trying to understand the ancient climate. So there are all kinds of ways that we can make the model more sophisticated.

The results of our quick and dirty model seem to be pointing in a certain direction: that if you do let clouds stabilize the climate, you keep it cooler. Then the oceans could have lasted a lot longer. Although the model is very simple, the results are sufficiently interesting to motivate us to go back and spend time to do a more sophisticated model. We need to try to get a handle on what the physical conditions were like during this interesting time when Venus was still holding on to its oceans.